The present invention relates to a method of producing epitaxial wafers exhibiting high gettering effect by doping a silicon single crystal with nitrogen in the process of growing thereof, slicing silicon wafers from the thus-grown single crystal ingot, subjecting the wafers obtained to predetermined heat treatment and then treating them for epitaxial growth.
In recent years, the tendency toward higher degree of integration of silicon W semiconductor integrated circuit devices has been rapidly increasing and, accordingly, silicon wafers on which devices are formed have been subjected to increasingly severe quality standards. Thus, since circuits become finer or thinner with the increase in integration density, crystal defects, such as dislocations, and metal element impurities other than a dopant in the so-called device active region where devices are formed on a wafer are subjected to more rigorous limitations than ever before, for such defects and impurities will cause increases in leakage current and/or shorten the life of a carrier.
In the prior art, a wafer sliced from a silicon single crystal obtained by the Czochralski method (hereinafter referred to as xe2x80x9cCZ methodxe2x80x9d and such wafer as xe2x80x9cCZ waferxe2x80x9d) is used in producing a semiconductor device. This wafer generally contains interstitial oxygen at a supersaturated concentration of about 1018 atoms/cm3. While oxygen is effective in preventing the occurrence of dislocations and thereby enhancing the silicon wafer strength and in providing a gettering effect, it is well known that oxygen deposits in an oxide form and thus induce crystal defects such as dislocations and/or stacking faults upon heating during device production.
Meanwhile, in the process of device production, the wafer is maintained at a temperature as high as 1,100 to 1,200xc2x0 C. for several hours for the formation of a field oxide film by LOCOS (local oxidation of silicon) or the formation of a well diffusion layer and, as a result, a defect-free denuded zone with a thickness of about several tens of micrometers is formed near the wafer surface owing to diffusion of interstitial oxygen to the outside. This denuded zone serves as a device active region and a condition with reduced crystal defects is thus provided.
However, the employment of the high-energy ion implantation method for well formation and a temperature of not higher than 1,000xc2x0 C. for device production in conjunction with device miniaturization corresponding to the increasing density of integration makes it difficult to allow the above-mentioned oxygen diffusion to a sufficient extent and thereby form a denuded zone in the vicinity of the surface. Thus, attempts have been made to reduce the oxygen content in the wafer. However, the formation of crystal defects cannot be suppressed to a satisfactory extent but oxygen reduction rather causes deterioration in wafer performance characteristics; no satisfactory results have thus been obtained. Therefore, an epitaxial wafer produced by allowing an epitaxial Si layer substantially free of crystal defects to grow on a silicon wafer has been developed and is widely used in producing a high integration density device.
However, even when an epitaxial wafer highly perfect as a crystal is used, the chance of contamination of the epitaxial layer by metal element impurities in the process of device production increases and the influence thereof also increases since the device process becomes complicated with the increasing degree of integration.
To cope with this contamination problem, there is the technique of gettering. This is the technique comprising collecting or capturing contaminant impurity elements at sites (sinks) outside the device active region to thereby avoid their adverse effects. One gettering technology is the so-called intrinsic gettering (IG) which uses oxygen-induced crystal defects (bulk micro defects; BMDs) induced during heat treatment in the device process as sinks.
In the case of an epitaxial wafer, however, the temperature reaches a level as high as 1,050 to 1,200xc2x0 C. in the step of epitaxial layer formation, so that oxygen precipitates to serve as nuclei of micro defects in the wafer reduce in size or disappear. It is thus difficult to induce a sufficient number of BMDs to serve as sources of gettering in the wafer in the subsequent device process. In particular when the device process is carried out at a lower temperature, as mentioned above, the growth of oxygen precipitates becomes slow and a problem arises in that no sufficient gettering effect upon metal impurities can be expected not only in the initial stage of the device process but also throughout the device process.
Therefore, extrinsic gettering (EG) has been used combinedly as an alternative gettering method. This method comprises introducing crystal defects by causing distortion by means of extrinsic factors such as sandblasting, grinding, laser irradiation, ion implantation, and growth of a Si3N4 or polycrystalline Si film on that side of the wafer which is reverse to the side on which devices are formed. The combined use of the EG method causes not only the increased cost problem due to the increase in the number of steps but also such problems as particle generation resulting from detachment of silicon fragments from the distorted layer and deterioration in flatness due to the growth of a polycrystalline silicon film.
To overcome the above problems, a method has been proposed which does not depend on the combined use of EG but improves the IG capacity itself by doping with an impurity capable of promoting oxygen precipitation in the step of single crystal growth. For example, Japanese Patent Application Laid-Open No. 11-189493 proposes a method which selects nitrogen as the element to promote oxygen precipitation and thus provide gettering effect and according to which the doping is carried out to a nitrogen concentration of not less than 1xc3x971013 atoms/cm3 to thereby form, within the wafer, stable precipitates which will hardly disappear even in the epitaxial step involving high temperature treatment.
However, when nitrogen doping is carried out to a specified concentration of not less than 1xc3x971013 atoms/cm3 according to the proposed method, nitrogen segregation occurs from the top to the tail of the single crystal ingot grown and thus the nitrogen concentration varies all over the whole length of the single crystal ingot. The BMD density, which influences the gettering effect, varies accordingly, hence no uniform gettering effect can be expected from the top to the tail of the single crystal ingot.
Further, Japanese Patent Application Laid-Open No. 2000-044389 proposes a method of increasing the gettering effect which comprises using a CZ wafer doped with 1xc3x971010 to 5xc3x971015 atoms/cm3 of nitrogen and subjecting the same, before epitaxial growth treatment, to heat treatment at a temperature not lower than 900xc2x0 C. but not higher than the melting point of silicon, desirably 1,100xc2x0 C. to 1,250xc2x0 C. Allegedly, this method can suppress the formation of defects within the epitaxial layer and can bring about a gettering effect. However, even this proposed method does not take into consideration the nitrogen segregation occurring from the top to the tail of the single crystal ingot grown, and it is difficult, by this method, to obtain a uniform gettering effect all over the single crystal ingot.
The present invention has been made in view of the problem that epitaxial wafers differ in gettering effect depending on the site of slicing from a single crystal ingot owing to nitrogen segregation on the occasion of growth by the CZ method. Thus, it is an object of the present invention to provide a method of producing epitaxial wafers having an equal and high level of gettering effect irrespective of the site of slicing from a single crystal ingot while suppressing the formation of defects in the epitaxial layer.
Among micro defects of a crystal which are caused by a content of oxygen, there are oxidation-induced stacking faults (hereinafter referred to as xe2x80x9cOSFsxe2x80x9d for short). These are stacking faults appearing in the substrate crystal covered with oxide film in the step of high-temperature thermal oxidation treatment during the device production process.
As mentioned above, a single crystal ingot grown by the CZ method contains oxygen and, therefore, when a wafer sliced therefrom is subjected to 1 to 20 hours of thermal oxidation treatment at 1,000 to 1,200xc2x0 C., ring likely distributed oxidation-induced stacking faults (hereinafter referred to as xe2x80x9cOSF ringxe2x80x9d) may occur on the crystal surface of the wafer in certain instances.
The OSFs are induced by the above-mentioned thermal oxidation treatment at 1,000 to 1,200xc2x0 C. with stable oxygen precipitates, which hardly disappear even at a high temperature of 1,200xc2x0 C. or above, serving as nuclei. The ring-like region within the wafer surface where OSFs are supposed to appear when such thermal oxidation treatment is carried out is herein referred to as a xe2x80x9cpotential OSF ringxe2x80x9d. When an epitaxial layer of Si is formed on the wafer surface including this potential OSF ring, the oxygen precipitate nuclei will not disappear but form BMDs in the device process after epitaxial layer formation, whereby an effective gettering effect is produced.
The OSF ring generally has a width of several millimeters to scores of millimeters, and the width of the OSF ring can be increased by nitrogen doping in the step of single crystal growth. Wafers sliced from this single crystal ingot show good gettering effect.
However, the density of oxygen precipitates, which serve as nuclei for OSFs, depends on the nitrogen concentration. Further, nitrogen is introduced into the single crystal ingot from the melt according to the segregation coefficient. Therefore, the nitrogen concentration differs between the top and tail of the ingot, hence the BMD density varies accordingly. In other words, in the single crystal ingot, the nitrogen concentration increases from the top toward the tail and the gettering effect varies accordingly. Thus, the gettering effect differs depending on the site of wafer slicing from the single crystal ingot.
When a CZ wafer is used, the growth of small precipitation nuclei can be promoted by subjecting the wafer to heat treatment at a temperature of not lower than 700xc2x0 C. but lower than 900xc2x0 C. prior to epitaxial growth treatment. Therefore, even such precipitation nuclei that may disappear upon high-temperature epitaxial growth treatment without this heat treatment can be grown by this heat treatment and, thereby, the density of precipitation nuclei which will not disappear upon epitaxial growth treatment but remain can be increased.
When this heat treatment is carried out, it is desirably carried out before the step of mirror polishing of wafers to be subjected to epitaxial treatment so that surface flaws appearing upon heat treatment, for example flaws caused by a wafer boat for supporting a wafer to be subjected to heat treatment, may not remain.
The heat treatment carried out prior to epitaxial growth treatment is intended to form oxygen precipitates which hardly disappear even in the epitaxial step. However, when the heat treatment time is longer than 4 hours, the growth of stacking faults may extend into the epitaxial layer, leading to a tendency toward formation of defects in the epitaxial layer. Therefore, it is desirable that the heat treatment time be not longer than 4 hours.
During the heat treatment, contamination may occur from the furnace. For preventing such wafer contamination, it is effective to form an oxide film as a protective film on the wafer. Thus, a mixed atmosphere composed of oxygen and an inert gas is desirably used as the atmosphere in carrying out the heat treatment. This mixture of oxygen gas in atmosphere only at wafer loading is also effective for the formation of oxide film.
When the above heat treatment is carried out prior to the step of mirror polishing of wafers, the oxide film formed on the wafer surface can be removed in the mirror polishing step. Therefore, it is not necessary to employ a particular step of removing the oxide film, for example a step of removing the oxide film with hydrofluoric acid (HF).
When the heat treatment prior to epitaxial growth treatment is carried out at a temperature in the vicinity of 900xc2x0 C., which is the upper limit, the BMD density in the body portion near the tail of the single crystal ingot becomes lower as compared with the other portions. This is caused by the fact that, in the conventional CZ method, when tailing is started, the pulling rate of the single crystal ingot is increased to 1.1 to 1.3 times the pulling rate of the body.
Thus, the body portion followed by the tail is cooled rapidly upon increasing the pulling rate. In the corresponding body portion, the time for the passage of the temperature range of 1,050xc2x0 C. to 700xc2x0 C. in which oxygen precipitation nuclei are formed is shortened as compared with the other body portions, whereby the formation of oxygen precipitation nuclei is inhibited.
The tail of a single crystal ingot can also be formed by carrying out tailing while raising the melt temperature, not by controlling the pulling rate. Thus, when the pulling rate of the single crystal ingot is not increased in starting tail formation but the tail formation is effected by raising the melt temperature, the number of oxygen precipitates can be increased and the BMD density can be leveled in the body portion close to the tail as well.
The above findings have led to completion of the present invention, which is directed to the methods of producing epitaxial wafers as specified below.
(1) A method of producing epitaxial wafers which comprises subjecting a silicon wafer sliced from a single crystal ingot grown by doping with not less than 1xc3x971013 atoms/cm3 of nitrogen to 15 minutes to 4 hours of heat treatment at a temperature not lower than 700xc2x0 C. but lower than 900xc2x0 C. and then to epitaxial growth treatment.
It is desirable that the above single crystal ingot have an oxygen concentration of not less than 11xc3x971017 atoms/cm3.
(2) In the above production method (1), the heat treatment is desirably carried out prior to the step of mirror polishing of wafers. Furthermore, a mixed atmosphere composed of oxygen and an inert gas is desirably used as the atmosphere in this heat treatment. As an inert gas, nitrogen gas or argon gas is desirable.
(3) In growing the above single crystal ingot, it is desirable that the pulling rate be not increased in starting tail formation as compared with the pulling rate of the body. Furthermore, it is desirable that, in the body portion from the boundary between the body and tail of the single crystal ingot to 200 mm above the boundary, the cooling rate from 1,050xc2x0 C. to 700xc2x0 C. be not higher than 2.5xc2x0 C./minute.